Counterbeam fast ignition experiments and the related studies

Y. Kitagawa1, Y. Mori1, Y. Nishimura1,2, K. Ishii1, R. Hanahaya1, S. Nakayama1, T. Sekine3,

N. Sato3, T. Kurita3, T. Kawashima3, H. Kan3, O. Komeda4, T. Nishi5, T. Hioki6, T. Motohiro6,

H. Azuma7, A. Sunahara8, Y. Sentoku9, E. Miura10, Y. Arikawa11, Y. Abe11, S. Ozaki12

1The Graduate School for the Creation of New Photonics Industries, Hamamatsu, Japan,2Toyota Technical Development Corp., Toyota, Japan,

3Hamamatsu Photonics, K. K., Hamamatsu, Japan,4Advanced Material Engineering Div., TOYOTA Motor Corporation, Susono, Japan,

5TOYOTA CENTRAL R&D LABS, INC., Nagakuate, Japan, 6Nagoya University, GREMO,

Nagoya, Japan, 7Aichi SR Center, Seto, Japan, 8Institute for Laser Technology, Osaka, Japan,9University of Nevada, Reno, USA, 10National Institute of Advanced Industrial Science and

Technology, Tsukuba, Japan, 11Institute of laser Engineering, Osaka University, Suita, Japan,12National Institute for Fusion Science, Toki, Japan

Introduction

　 The recent key physics of the laser fusion is how to make hot sparks for the alpha burning

in the dense core. The fast ignition is expected to form a hot spark even with a non-uniform

illumination configuration, such as with a counter-illumination one. A kJ-class mini reactor

CANDY, as in Fig. 1(a), is proposed for an engineering feasibility study of the power plant in the

counter beam fast ignition scheme fusion. To develop CANDY, we are performing fast ignition

experiments and are developing a high-repetitive pellet injection using a high-repetition-rate

laser-diode(LD)-pumped laser system HAMA with counter beam configuration[1]. Figure 1(b)

shows a roadmap through CANDY to the Power plant.

(a) (b)

Figure 1: (a) Image of Mini reactor CANDY. (b) Roadmap through CANDY to the Power plant.

Counter-heating of CD Double foils

　 First, using double-foil CD targets, we have proposed a compact fast ignition experiment

to initiate a fusion reaction and clarify its dynamics[2]. A 4 J/0.4-ns output of an LD-pumped

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high-repetition laser HAMA is divided into four beams, two of which counter illuminate double

foils separated by 100 µm for implosion, as shown in Figs. 2[1]. The remaining two beams,

compressed to 110 fs for fast heating, illuminate the same paths. Hot electrons produced by

the heating pulses heat the imploded core, emitting x-ray radiations >40 eV and yielding 103

thermal neutrons. Simultaneously the driven shock waves seem to enhance the core emissions.

We discovered that the hot electrons reach the core plasma to emit x-ray radiations, and also

that they deposit their energy to deuterons close to the ablation surface, driving a shock wave to

heat the core. Once heated, the core plasma maintains a temperature of a few tens eV as long as

the core stagnates. PIC simulations suggest that fast heating occurs due to the Weibel instability,

as in Fig. 2(g) with on-side illumination and (h) with counter illumination.

(d) (e)

(f)(h)

(g)

Figure 2: (a) Counter-illumination scheme and (b) Double foil CD target. (c) STAR 1D hydrodynamic

flowchart. Laser: 5×1013 W/cm2. 2ω 100-fs probe captures the double foil image (d) before the shot and

(e) when the imploded plasmas collide with each other and form the core. The gap separation is 100 µm.

(f) X-ray streak image of heating of the imploding foils at 1.3-ns delay. Three bright points are shown at

the central core and at the peripheral (side) area. (g) 2D PIC of magnetic fields for left side(one-side)

illumination, and (h) for counter(two-side) illumination.

Counter-heating of imploded CD shells

　A tailored-pulse-imploded core with a diameter of 70 µm was flashed by counter irradiat-

ing 110 fs, 7 TW laser pulses[3]. Photon emission ( 40 eV) from the core exceeds the emission

from the imploded core by six times, even though the heating pulses energies are only one-

seventh of the implosion energy. The enhancement of photon emission shows the coupling from

the heating laser to the core to be 14 %. Neutrons were produced by counter propagating fast

deuterons accelerated by the photon pressure of the heating pulses. A collisional 2D PIC reveals

that the two counter-propagating fast electron currents induce mega-Gauss magnetic filaments

in the center of the core due to the Weibel instability. The counter-propagating fast electron cur-

rents are absolutely unstable and independent of the core density and resistivity. Fast electrons

with energy below a few MeV are trapped by these filaments in the core region, inducing an

additional coupling. This leads to the bright photon emissions.

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(c)

(d)

Figure 3: (a) Tailored pulse chain, (b) Implosion diagram: radius of fluid elements versus time, (c) x-

ray streak images without heating pulses and (d) with heating pulses. (e) Electron energy distribution

in the core integrated in the region between X = 55 µm and 65 µm at 500 fs. Solid(red), dotted(blue),

and dashed(black) lines indicate the data for the cases of counter-propagating beams, one-beam, and

one-beam with two times the intensity and the energy, respectively[3].

Counter-beam engagement of injected targets

　 The pellet injection, as well as the repetitive laser illumination, is a key technology for

realizing the inertial fusion energy. By counter beam illumination, we have first demonstrated

600× repetitive pellets injection and neutron generation[4]. Figure 4(a) CD bead pellets, af-

(a) (b) (c)

Figure 4: (a)Pellet injection system. Pellet loader stores more than 10,000 pellets. Rotating disk

has holes to catch and feed pellets to the exit hole above a laser focal point. Collector box

recollects the engaged pellets. (b) Visible emission from a engaged CD bead, and (c) snapshot

of pellet at the instance of engagement by 2 ω harmonic laser probe[4].

ter free-fallen for a distance of 18 cm at 1 Hz, are engaged by the two counter laser beams

from laser-diode-pumped ultra-intense laser HAMA[1]. The laser energy on target, pulse du-

ration and wavelength are 0.63 J per beam, 104 fs and 800 nm, respectively. The intensity is

9.4× 1019 W/cm2. They produced D(d,n)3He reacted neutrons (2.45 MeV DD neutrons) with

the yield of more than 5.0×104/4π sr. The laser is first found to bore a straight channel through-

out the 1 mm-diameter beads. The results of the pellet injection, the repetitive laser illumination

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and hole boring, are the useful technologies and findings for the next step to realize inertial

fusion energy.

Ion Fast-ignition Experiments using the LFEX at Osaka

　 By using a single-shot kJ laser with counter beam configuration, we have demonstrated a

fast-ignition fusion[5]. Figure 5 shows that the counter (two side) blue beams from the GEKKO

XII lasers implode a core, to which then the LFEX ( the red beam) is illuminated as close as

possible. We found that the energetic ions heat the core rather than the hot electrons. LFEX

illumination has enhanced DD neutron yields by a factor of 1000 (5× 108 n /4π sr), the best

ever obtained in fast-ignition scheme. 6×107 n /4π sr thermal neutrons are observed at the same

time, which is also the fast-ignition scheme record. STAR1D hydrocode predicts that deuterons

are related to beam fusion. Hot electrons heat the entire core from 800 eV to 1 keV, whereas

ions including carbon+6 pre-dominantly heat the region 20 µm from the core surface from 1 to

1.8 keV. The scheme is a potential path to fast-ignite the core at high gain fusion.

(b) (c)(a)

Figure 5: (a) Two-side GXII beams (blue beams) preimplode a core and then LFEX (red beam) heats

ions in the core. (b) X-ray pinhole image of the core before LFEX shot, (c) after shot[5].

Conclusion

　We proposed two kinds of the core heating. The ultraintense laser driven hot electrons

effectively heat the core with the counter beam configuration. The ultraintense laser driven ions

heat the core, when the laser irradiates directly the core. The counterbeam configuration must

make the fast ignition a potentially fruitful scheme for the inertial confinement fusion.

References

[1] Y. Mori et al. Nuclear Fusion 53 073011 (2013)

[2] Y. Kitagawa et al. Phys. Rev. Lett. 108 15 5001(2012)

[3] Y. Mori et al. to be published in Phys. Rev. Lett. 2016

[4] O. Komeda et al. Scientific Reports 3 2561 (2013)

[5] Y. Kitagawa et al. Phys. Rev. Lett. 114 195002 (2015)

43rd EPS Conference on Plasma Physics P4.095